1. Field of the Invention
The present invention relates to a position measurement apparatus, an imaging apparatus, and an exposure apparatus, which can manufacture a device having a micro pattern, such as an a semiconductor chip (e.g., an integrated circuit (IC) or a large scale integration (LSI)), a liquid crystal panel, a charge coupled device (CCD), a thin-film magnetic head, and a micro machine.
2. Description of the Related Art
A conventional reduced projection exposure apparatus (stepper), capable of manufacturing semiconductor devices, requires a high-accurate technique for capturing an image of a mark formed on a wafer or a reticle and detecting a position of the mark based on a signal waveform obtained from the captured image.
A conventional method for capturing a mark image is described below. FIG. 8 illustrates a conventional exposure apparatus usable in the manufacturing of semiconductor devices. In FIG. 8, “R” represents a reticle (i.e., an original plate for exposure use), “W” represents a wafer (i.e., a substrate to be exposed), and “WM” represents a wafer mark (i.e., a mark to be observed). A projection optical system 1 has an optical axis parallel to a z-axis of the xyz-coordinate system. A mark imaging optical system S includes an alignment illumination unit 2, a beam splitter 3, two imaging optical systems 4 and 5, and an imaging unit 6. Furthermore, the conventional exposure apparatus includes an analog/digital (A/D) conversion circuit 7, an integrating circuit 8, an image processing circuit 9, a stage driving unit 10, a movable stage 11 causing a three-dimensional motion, and a stage position measurement unit 12 (e.g., an interferometer).
The conventional exposure apparatus captures an image of the wafer mark WM in the image area WP according to the following procedure. First, the stage driving unit 10 moves the stage 11 to a position where the stage position measurement unit 12 can observe the mark WM on the stage 11. Next, the alignment illumination unit 2 emits exposure light (luminous flux) that reaches the wafer mark WM via the beam splitter 3, the reticle R, and the projection optical system 1. FIG. 2A illustrates an exemplary wafer mark WM which includes a plurality of same lattice patterns. The luminous flux reflects on the wafer mark WM and returns to the beam splitter 3 via the projection optical system 1 and the reticle R. Furthermore, the luminous flux reflects on the beam splitter 3 and, via the imaging optical system 5, forms an image of the wafer mark WM on an imaging plane of the imaging unit 6.
The imaging unit 6 applies photoelectric conversion to the image of the wafer mark WM. The A/D conversion circuit 7 converts the image signal into a two-dimensional digital signal sequence. The integrating circuit 8 receives the two-dimensional digital signal sequence from the A/D conversion circuit 7 and integrates the received digital signal sequence in the Y-direction of FIG. 2A. In other words, the integrating circuit 8 converts the two-dimensional digital signal into a one-dimensional digital signal sequence S0(x) as illustrated in FIG. 2B. The image processing unit 9 measures a central position of the wafer mark WM based on the converted digital signal sequence, or measures a contrast value as an index for searching a focal position of the optical system.
The above-described mark imaging method is effective when an apparatus requires an accurate waveform of a mark signal. However, as illustrated in FIG. 3A, an x-axis, y-axis or z-axis position of the stage 11 fluctuates during a mark image capturing operation. The position may vibrate or move away from the initially set position (x-axis position x0, y-axis position y0, or z-axis position z0).
Accordingly, the integrating circuit 8 cannot generate an ideal digital signal sequence S0(x) illustrated in FIG. 2B, and generates a deformed signal sequence S1(x) due to fluctuation of the stage 11 as illustrated FIG. 2C. The deformed signal sequence S1(x) may induce measurement errors in image processing, such as a contrast measurement or a pattern matching.
To solve the above-described problem, as discussed in Japanese Patent Application Laid-Open No. 6-36990 or in Japanese Patent Application Laid-Open No. 2003-203839, there is a conventional method for correcting an alignment measurement value or a deformation value of the digital signal sequence based on a continuously monitored stage position during an image accumulation operation.
However, as illustrated in FIG. 3B, the luminous intensity of the alignment illumination unit 2 may fluctuate during an image accumulation operation. In this case, the above-described conventional method cannot accurately correct an alignment measurement value or a deformation value of the digital signal sequence. When the luminous intensity of the alignment illumination unit 2 has a peak value in a temporal distribution, or when the illumination unit 2 emits pulsed light, similar problems may arise.